RESPONSE OF RICE VARIETIES TO SOIL-SALINITY AND AIR HUMIDITY - A POSSIBLE INVOLVEMENT OF ROOT-BORNE ABA

In a phytotron experiment four rice varieties (Pokkali, IR 28, IR 50, IR 31785-58-1-2-3-3) grown in individual pots were subjected to low (4 0/55% day/night) and high (75/90%) air humidity (RH), while soil salin ity was gradually increased by injecting 0, 30, 60 or 120 mM NaCl solu tions every two days. Bulk root and stem base water potential (SWP), a bscisic acid (ABA) content of the xylem sap and stomatal resistance (r s) of the youngest fully expanded leaf were determined two days after each salt application. The SWP decreased and xylem ABA and rs increase d throughout the 8 days of treatment. The effects were amplified by lo w RH. A chain of physiological events was hypothesized in which high s oil electric conductivity (EC) reduces SWP, followed by release of roo t-borne ABA to the xylem and eventually resulting in stomatal closure. To explain varietal differences in stomatal reaction, supposed cause and effect variables were compared by linear regression. This revealed strong differences in physiological reactions to the RH and salt trea tments among the test varieties. Under salt stress roots of IR 31785-5 8-1-2-3-3 produced much ABA under low RH, but no additional effect of low RH on rs could be found. By contrast, Pokkali produced little ABA, but rs was strongly affected by RH. RH did not affect the relationshi ps EC vs. SWP and SWP vs. ABA in Pokkali, IR 28, and IR 50, but the re lationship ABA vs. rs was strongly affected by RH. In IR 31785-58-1-2- 3-3 RH strongly affected the relationship SWP vs. ABA, but had no effe ct on ABA vs. rs and EC vs. rs. The results are discussed regarding po ssible differences in varietal stomatal sensitivity to ABA and their i mplications for varietal salt tolerance.